This invention relates to a switching apparatus with an electro-mechanical adjusting drive, of the type including means for the linearization of the adjusting movement of an element of a first adjusting drive in relation to an electrical voltage and a supply voltage producing device which has a control inlet and an outlet for a supply voltage, which is connected with the adjusting drive.
Adjustment drives, particularly piezoelectrical adjustment drives, which are frequently also termed "activators" or "translators", have the disadvantage that their deflection is not linear to the electrical signal which is applied. This non-linearity is particularly disadvantageous, therefore, because it is not constant in time. It can consequently also not be compensated with a coefficient rate which is, to be sure, adjusted individually, but which however, is constant in time. The temporal dependence is based on hysteresis phenomena which depend on the previous history of the course of the voltage on the piezo-element. This hysteresis behavior is not temporally constant but has, rather, a consequence on the basis of a subsequent polarization of the ceramic. There is, in addition, a temporal dependence on the temperature.
A switching apparatus of the type under discussion, with a piezo-electrical adjusting drive in which the means for the linearization of the adjusting movement, which should be independent of the hysteresis, the drift and the temperature, have a measuring system, by means of which the expansion of the piezoelectrical adjusting element is measured, and a corresponding measuring signal, which is superimposed as an ACTUAL-value in a closed control system over a THEORETICAL-value, so that the supply voltage for the piezoelectrical adjusting element changes in the sense of a counter-coupling, and the course of the deflection is linearized to the ACTUAL-value, is already known from the publication "Products for the Micro-Adjustment Technology" by the firm "Physik Instrumente GmbH & Co.,", in Waldbronn.
Expansion measuring strips which adhere to the piezoelectrical element, and which are positioned within a bridging circuit, serve as the measuring system. In another known arrangement of the type which is under discussion (company publication "Queensgate System 2000 Nano-Positioning" by the firm Queensgate Instruments in Berkshire, SL5 7PW, England), the determination of the ACTUAL-position of the piezoelectrical adjusting element is carried out by means of the capacitative measurement of the interval.
Both of these known solutions are time-consuming, expensive and increase, in particular, the volume of the adjusting drive, since the device for the measurement of the ACTUAL-position of the piezoelectrical adjusting element is directly attached to the same.
A positioning device for a cutting edge of a machine tool is already known from JP 64-35613 (A). The cutting tool is held by means of a piezoelement, with the help of which, through the application of a corresponding electrical voltage, the position of the cutting edge of the machine tool can be adjusted, in order to thereby determine the depth of the cutting. By means of a second piezoelectrical element with the same characteristics, an error signal is produced, which is added to the control signal for the adjustment of the cutting edge of the cutting tool, in order to compensate for a dynamic hysteresis of the piezoelement which is adjusting the cutting edge. The linearization of the dependence of the change of the length of the piezoelement holding the cutting tool to its supply voltage is not thereby attained as the result.
The task which forms the basis of the invention is that of creating a switching apparatus of the type under discussion, in which the connection of the adjusting movement of the adjusting element with an electrical voltage, and thereby the linearity, is improved so that more precise conclusions can be made, from the voltage, in regard to the movement of the adjusting element which has actually taken place. The switching apparatus should, furthermore, be simple and inexpensive. The volume of the adjusting drive should not thereby be increased.
The task which forms the basis of the invention is solved by means of the theory which is stated in the characterizing portion of patent claim 1, i.e., providing the means for the linerization with a second adjusting drive which is connected with the outlet of the supply voltage producing device and a first length measuring device is provided which scans the adjusting movement of the adjusting element of the second adjusting drive and produces an output signal which is connected in a linear manner with the adjusting movement of the adjusting element of the first adjusting drive.
The concept which lies at the basis of the invention is that of providing a second adjusting drive, which is stressed with the same supply signal as the first adjusting drive, and consequently carries out the same movements as the first adjusting drive. The movements of this second adjusting drive are measured by means of an electrical length measuring device, the output voltage of which is linearly dependent on the position of movement of the adjusting element of the second adjusting drive. It is consequently also linearly connected with the adjusting movement of the first adjusting drive. It can thus serve as a corresponding reference voltage. It can, for example, be indicated immediately, so that the position of movement of the adjusting element of the adjusting drive is then indicated immediately. The linear connection is then particularly provided if the second adjusting drive is constructed in the same manner as the first adjusting drive. It thus exhibits the same mechanical-electrical behavior. It is of particular advantage that the first electrical length measuring device is not connected with the first, and preferably piezoelectrical, adjusting drive, or has to be positioned in the immediate vicinity of the same. The first adjusting drive is thus completely free of additional elements, so that its volume is not increased by means of additional components. The electrical connection of both of the adjusting drives thus represents no increase of the volume.
If, in addition, the second adjusting drive can also be positioned, with the corresponding length measuring device, in a given place, then it is, as a rule, appropriate to position it as close as possible to the first adjusting drive, so that both adjusting drives are subjected to the same temperature influences. The lineraity which is thus attained is consequently also independent of the influences of temperature.
The basic concept of the invention can be additionally formed in various manners. In accordance with the solution which is characterized in patent claim 3, the switching apparatus is further formed as a device for the scanning of the surface of a test piece. The adjusting element of the first adjusting drive moves the test piece in parallel to its surface, so that a second length measuring device scans the contour of the surface of the test piece and produces an electrical output signal which is fed, as a "Y"-signal, into a writing or viewing device, the "X"-signal of which is formed by the output signal of the first length measuring key switch.
Because of the linear condition of the output voltage of the first length measuring key switch with the adjusting movement of the adjusting element of the first adjusting device with the writing or viewing device, the surface contour of the test piece, scanned by the second length measuring device, is written or made visible in the "X"-direction in an undistorted manner.
One other possibility for the further formation of the concept of claim 1 is characterized in patent claim 4, in accordance with which, with the inclusion of the second adjusting drive and the corresponding length measuring device, a control system is formed, on the basis of which the dependence of the movement of the adjusting element of the second adjusting drive is linearized in relation to the control voltage. Since, however, both adjusting drives, which are preferably constructed alike, are stressed with the same supply voltage, it follows from this that the movement of the adjusting element of the first adjusting drive is also linearly connected with the control voltage which is supplied. It is thus possible to record a length (in the direction of movement of the adjusting element or transversely to the same), which is scanned by the surface of a test piece moved by the adjusting element of the first adjusting drive, in linear dependence on the control voltage which is fed in (such as, for example, a THEORETICAL value).
This further formation is particularly suitable in its application in the scanning of a surface contour, if this scanning should take place with a peak, the distance of which should be kept constant in relation to the surface during the movement along the surface, such as is necessary, for example, in the case of scanning microscopes (PHYSICAL REVIEW LETTERS, 1986, pages 930-933), or in acoustical scanning microscopes (DE 38 20 518 C1). In accordance with the invention, there is provided for this purpose a second control system, in which the electrical measuring signal corresponding to the contour of the surface which is measured, in the sense of a counter-coupling, is added to the control signal, which controls the supply voltage for the preferably piezoelectrical adjusting drives.
Additional advantageous configurations are presented in patent claims 5 and 6.
The electrical length measuring devices which are used in accordance with the invention can essentially be of any type that is desired. This may involve inductive, capacitative, interferometric, or other length measuring devices which emit an electrical measuring signal.